Cathode coating for electron discharge devices



June 14, 1949. o. s. BRIGHT, JR 2,473,358

CATHODE COATING FOR ELECTRON DISCHARGE DEVICES Filed Aug. 23, 1946 l-vs-ron ORLOW S. BR/aHz' JR.

Patented June 14, 1949 CATHODE COATING FOR ELECTRON DISCHARGE DEVICES Orlow S. Bright, Jr., Canton, Mass, assignor to Raytheon Manufacturing Company, Newton, Mass, a corporation of Delaware Application August 23, 1946, Serial No. 692,462

Claims. 1

This invention relates to electron-discharge devices, and more particularly to thermionic gaseous discharge rectifiers in which the cathode is heated to the temperature of thermionic emission solely by means of the discharge current.

An object of this invention is to provide a cathode-coating bath whereby the life of the cathode of rectifiers of this kind may be increased.

Another object is to provide an electron-emissive coating which may be readily applied to cathodes.

A further object is to devise a cathode coating for tubes of .the above type which will enable initiation of the operation of such tubes at lower starting voltages.

A still further object is to provide an electronemissive coating on cathodes by means of which the emitting life of cathodes for tubes of the above type may be increased.

The foregoing and other objects of the invention will be best understood from the following description of some exemplifications thereof, reference being had to the accompanying drawing. wherein:

Fig. 1 is a detailed view, on an enlarged scale, of a coated coiled-coil cathode according to the invention;

Fig. 2 is a perspective view of a stem and electrode structure utilizing the cathode of Fig. 1; and

Fig. 3 is a vertical cross-section of a complete tube utilizing the cathode of Fig. 1.

Referring, now, to the drawing, and particularly to Fig. 1 thereof, a cathode coil for a thermionic gaseous discharge rectifier of the ionically-heated-cathode type is shown at I. This cathode is preferably of the coiled-coil or double coil type, comprising a plurality of relatively closely-spaced minor turns 2 of fine wire, for example, tungsten, wound on a suitable mandrel 3, the mandrel 3 with its winding of fine wire being wound. on a larger mandrel (not shown), which is later removed or dissolved, to provide a plurality of more-widely spaced major turns 4 constituting the cathode coil I. The entire external surface of cathode coil I, includingthe exposed portions of mandrel 3, is covered with an electron-emissive coa ing 5; this coating preferably does not connect the separate adjacent major turns 4, though it can and preferably does connect the separate adjacent minor turns 2.

It has been found that, when coating 5 is provided on the cathode coil I by direct application thereto of a mixture comprising both barium and strontium oxides, much longer cathode life is achieved than by the use of other known coatings; when the two oxides are present in equal percentages (that is, in equal parts by weight), the activity or emissivity is greatest. Also, it has been found that, if a mixture or suspension of both said oxides is used as a coating bath, the coating produced adheres better than do other known coatings. In one mode for carrying out this invention, the cathode coil I is dipped in a mixture, comprising a suspension of fifty per cent by weight of barium oxide and fifty per cent by weight of strontium oxide, in a refined mineral oil vehicle, after which said coil is removed from the mixture and electric current is passed therethrough to heat said coil to thereby fuse the'coating 5 to cathode coil I. Since the oxide coating 5 is highly hygroscopic, the coil I should be protected from the air, for example, by immersion in a protective liquid such as a petro leum distillate, from the time coil I is coated until it is ready to be mounted in the gaseous discharge rectifier envelope.

The above oxide suspension also ordinarily includes a trace of dimethyl phthalate, to act as a plasticizer, and sometimes also includes a trace of barium hydroxide, in order to lower the fusion point of the mixture, or to give the proper fusion temperature.

It has also been found that, in some cases, the above mixture of barium and strontium oxides alone provides an emissive coating which causes the gaseous discharge tube to have a starting voltage which is undesirably high. In order to provide a coating which will lower the starting voltage of the tube, thorium .oxide is added to the coating bath or mixture of strontium and barium oxides, a bath having the following ercentages by weight having been found highly effective: thorium oxide 15 per cent, barium oxide 42% per cent, and strontium oxide 42% per cent, this mixture being suspended in a vehicle of refined mineral oil as before, and havin added thereto a trace of dimethyl phthalate as a plasticizer, and if required a trace of barium hydroxide to lower the fusion point of the mixture. As before, the cathode coil I is dipped in this mixture to provide thereon an electron-emissive coating 5, after which, as before, electric current is passed through the coil to heat the same 'to thereby fuse the coating 5 to said cathode coil. After the coil I has been coated, it is maintained submerged in a protective liquid, such as a purified kerosene, until ready for assembly into a tube.

It is apparent, from the above-described coating process, that said process is relatively simple and is therefore readily applicable to quantity production. There are also advantages in the fields of longer cathode life and of lower tube starting voltages, which have been set forth above.

Now referring to Figs. 2 and 3, which show the structure of a thermionic gaseous discharge rectifier using a cathode coated in accordance with my invention, said rectifier consists of a glass envelope 6 having a reentrant stem I at the upper end of which is a press I. The tube may be filled with an ionizable gas at a pressure sufllciently high to produce copious ionization therethrough upon the passage of a discharge. I prefer to use a rare gas or a mixture of such gases at a pres-v sure of about four to five mlllirneters of mercury more particularly I prefer to use argon at the above pressure.

The press 8 is formed with a main portion 9 at the end of which are disposed the transverse wing members Ill. The wing members I extend from the ends of the main portion 9 at an angle thereto so as to provide supporting press members transversely beyond the ends of the main portion 9.

The rectifier is provided with two anode structures ll each surrounded by a metallic anode shield [2. The, maximum distance between the interior of the shield l2 and the exterior of the anode structure is sufiiciently small so that a discharge will not ordinarily occur in the space between these elements. The lower ends of the anode structure II and of the shield l2 are sealed in the main stem portion 9. An anode lead I3 is connected to the lower end of each anode ll so as to provide an external electrical connection.

In order to support the cathode coil l of the rectifier, a cathode lead I4 is sealed in each of the wing members Ill. Due to the fact that the wings I'll extend outwardly from the ends of the main stem portion 9, the cathode leads l4 emerge from the wing members ill at a point where they lie transversely beyond the shields I2 of the anode structures l I. The opposite straight ends of cathode coil I are welded to cathode leads l4, one end being welded to each lead l4 and the coil I being mounted with the axis of the major turns 4 substantially horizontal.

The main press portion 9 may likewise be provided with a getter support or standard I! which supports a getter yoke l6. Any suitable getter II may be secured to the getter yoke for the wellknown purpose of gettering the tube. The reentrant stem 1 is likewise provided with the usual exhaust tube l8.

The tube described above may be provided with the insulating base IS in which are supported conducting base pins 20. The two anode leads I3 are connected to two of the base pins while the two cathode leads l4 are connected to one of the base pins 20. A sleeve 2| of insulating material is positioned in the hollow reentrant stem 1 this sleeve surrounds exhaust tube I8 and cathode leads I 4 and serves to prevent cathode leads I 4 and anode leads l3 from coming into contact with each other. The insulating base I9 is provided with a centering plug 22 having an orienting feather 23 formed thereon.

In order to prevent disturbances produced within the tube from escaping from the tube, said tube is provided with a metal shell 24 which is provided at its lower end with a ring portion 25. The shell 24 and ring portion 25 are maintained in place around the glass envelope 6, and also retain the insulating base I! in place with respect to said glass envelope 6 by having portions 25 of the ring portion 25 indented into corresponding recesses 21 in the insulating base IS. The metal shell 24 has connected thereto a conductor 2. which is likewise connected to one of the base pins 20, whereby the shell 24 is adapted to be connected to ground. It is desirable to resiliently support the glass envelope 6 within the metal shell 24, and for this reason a resilient cup 29 is interposed between the upper end of the glass envelope 6 and the corresponding upper walls of the metal shell 24. This resilient cup is preferably formed from a piece of paper made resilient, as by crin kiing the edges thereof. The lead-in wires l3, I4, and 28 possess in themselves a certain amount of resiliency so that the glass envelope 6 is resiliently supported within the surrounding shell structure by means of the lead-in wires as well as the resilient cup 29.

As described in LeVan Patent No. 2,273,054, dated February 17, 1942, the initiation of a discharge between the anodes and the cathode causes the cathode coil l to be heated to the temperature of thermionic emission. A tube of this type has what is known in the art as an ionically-heated cathode.

While I have described the specific details of some particular embodiments of my invention, it is, of course, to be understood that modifications may be made without departin from the spirit thereof. For example, other types of anodes and other shapes of cathodes might be employed. While I have shown two anodes in this embodiment, it is, of course, obvious that only one or more than two anodes might be employed. Furthermore, while I have described a cathode which is adapted to be heated by the energy of the discharge, my invention may be employed with cathodes which are heated by means external to the discharge, such as, for example, a current passed through said cathode. Numerous other modifications will be apparent to those versed in the art and therefore it is desired that the appended claims be given a broad interpretation commensurate with the scope of this invention within the art.

What is claimed is:

l. A coating bath for electron-emitting surfaces, comprising a suspension of the oxides of barium, of strontium, and of thorium, in a suitable vehicle, barium oxide and strontium oxide being present in said suspension in approximately equal percentages by weight.

2. A coating bath for electron-emitting surfaces, comprising a suspension consisting of the following percentages of compounds by weight: 15 per cent thorium oxide, 42 per cent barium oxide, and 42% per cent strontium oxide, in a suitable vehicle.

3. A gaseous-discharge device, comprising an envelope containing a low-pressure gaseous atmosphere, an ionically-heated cathode and one other electrode mounted in said envelope, said cathode having an emissive coating thereon which comprises a mixture of approximately equal percentages of barium and strontium oxides and a substantial quantity of thorium oxide.

4. A gaseous-discharge device, comprising an envelope containing a low-pressure gaseous atmosphere, an ionically-heated cathode and one other electrode mounted in said envelope, said cathode having an emissive coating thereon which comprises a mixture of 15 per cent by weight of thorium oxide, 42% per cent by weight of barium 332:, and 42% per cent by weight or strontium UNITED STATES PAW 5. A coating for electron emissive surfaces, Number Name Date I comprising the oxides of barium, strontium and 1,794, 8 Just Feb. 24, 1931 thorium, said oxides being present in the same 1,828,545 Ruben Oct; 20, 1931 order of magnitude of percentage by weight. 1,887,868 Cheney Nov. 15, 1932 1 REFERENCES CITED Number Country Data! The following references are of record in the Great 1923 file of this patent: 

